Not Applicable.
Not Applicable.
The present invention relates to joint prostheses, and more particularly, to joint prostheses which are implantable within the medullary canal of a bone.
Joint arthroplasty is a well known surgical procedure by which a diseased and/or damaged natural joint is replaced with a prosthetic joint. Joint arthroplasty is commonly performed for hips, knees, elbows, and other joints. The health and condition of the natural joint to be replaced dictate the type of prosthesis needed for implantation. A hip arthroplasty refers to a surgical procedure where the natural femur is re-sectioned and a femoral component is implanted within the medullary canal. In a total hip arthroplasty, the natural acetabulum is replaced with an acetabular component which provides an articulation surface for the head of the femoral component.
Successful implantation of a femoral component requires adequate initial fixation and long term stability of the prosthetic component. One technique to secure the femoral component within the medullary canal includes the use of bone cement. It has been found, however, that known bone cements have certain drawbacks. For example, the implanted prosthesis can loosen over time and ultimately necessitate surgical revision of the implant.
Other attempts to enhance the fixation of a press-fit prosthetic component in the medullary canal include the use of various surface features formed on the implant, such as uniformly spaced flutes. While such surface features may provide some fixation to bone, the overall geometry of the prosthetic implant is not optimized for the available dense cancellous bone. For example, it is known that the most dense cancellous bone in the proximal portion of a femur is generally located on the anterior-lateral and postero-lateral portions of the femur. However, implants having evenly spaced ribs do not take into account a bone density gradient in the cancellous bone so that initial implant fixation is not optimized. Initial implant fixation is important to minimize motion between the bone and the implant so as allow bone ingrowth onto the implant surface which increases the likelihood of long term implant fixation.
Further, the contour of some femoral components is such that high quality, dense cancellous bone must be removed in order to accommodate the implant. This may result in the surface features being implanted within cancellous bone of lesser density than that which was removed. In addition, some implants are formed so as to conform to the cortical bone so that any surface features are positioned without regard to cancellous bone density.
It would, therefore, be desirable to provide a prosthetic component having surface features in the form of bone engaging ribs with a geometry for achieving optimal fixation in dense cancellous bone upon implantation into the medullary canal of a bone.
The present invention provides a joint prosthesis having an overall contour and surface geometry which optimize fixation properties. Although the invention is primarily shown and described as a femoral component of a hip prosthesis, it is understood that the invention is applicable to other joint prostheses as well.
In one embodiment, an artificial hip femoral component for implantation into the medullary canal of a femur includes a plurality of longitudinal, bone-engaging ribs formed on anterior and posterior surfaces of the femoral stem. The ribs are sized and spaced for optimal fixation within predetermined dense cancellous bone. A desirable geometry, spacing and number of ribs can be determined based upon a variety of factors, such as cancellous bone density profile and implant size. In one embodiment, at least three longitudinal ribs are formed on the surface of the stem with the spacing between adjacent ribs increasing in one direction. The ribs provide initial implant fixation, minimize micromotion of the implant, and facilitate load transfer from the implant to the bone.